As I said before, the Ofcom thing is totally and utterly useless for me on multiple grounds: it has no detail at all and it makes test measurements of the Internet pipe
My understanding is that it makes some measurements fro the Internet pipe, others from just pinging the router locally. That is why I mentioned it. But check the white paper. Or ignore it, I'm not attempting to twist your arm.
Regarding what's possible in iOS, if an API is documented that is fine, dev's can use it in their Apps. If they use undocumented APIs, even if the App works, and Apple find out during approval then the App will likely rejected. Even if it is accepted, it could suddenly stop working at some future iOS version.
From a brief glance earlier today, may be wrong, but I got the impression attempts to access the negotiated speed would fall into 'undocumented API' territory, or at least a grey area. That may be why there is a shortage of Apps that do so.